As a preform of a fiber-reinforced composite material (hereinafter abbreviated as a composite material in some cases) using a carbon fiber, an aramid fiber, a glass fiber and the like as a reinforcing fiber, a random mat exhibiting isotropy is used because of formability and ease of process steps. This random mat can be obtained by a spray-up method (dry type) in which a cut reinforcing fiber alone or a thermosetting resin is sprayed simultaneously to a shaping-mold or a method (wet type) in which a reinforcing fiber cut in advance is added to slurry containing a binder and subjected to papermaking.
As a means for improving mechanical properties of the composite material, an increase of a reinforcing fiber volume content ratio (Vf) is known, but in the case of the random mat using cut fibers, it has been difficult to increase the reinforcing fiber volume content ratio due to reasons such as presence of fibers oriented in the three-dimensional direction, frequent entanglement of fibers and the like. Moreover, when the random mat is used, since the fibers are discontinuous compared to the case in which continuous fibers are used, strength of the reinforcing fibers cannot be sufficiently developed, and there was a problem that a development rate of strength of the reinforcing fiber after being made into a shaped product was 50% or less of a theoretical value. Non-Patent Literature 1 discloses a composite material made from a carbon fiber random mat including a thermosetting resin as a matrix. The development rate of strength such a composite material is approximately 44% of the theoretical value.
Moreover, a conventional composite material including a thermosetting resin as a matrix is obtained by heating/pressing for 2 hours or more an intermediate material called prepreg obtained by impregnating in advance a reinforcing fiber base material with a thermosetting resin using an autoclave. Recently, an RTM molding method in which a reinforcing fiber base material not impregnated with a resin is set in a metal mold and then, the thermosetting resin is poured is proposed, and molding time has been drastically reduced, but even if the RTM molding method is used, 10 minutes or more are required for molding one component.
Thus, a composite including a thermoplastic resin as a matrix instead of the conventional thermosetting resin attracts attention.
In a thermoplastic stamping molding (TP-SMC) (Patent Literature 1) including a thermoplastic resin as a matrix, a chopped fiber impregnated with a thermoplastic resin in advance is heated to a melting point or more, the impregnated chopped fiber is fed into a part of a mold, the mold is tightened immediately, and the fiber and the resin are made to flow inside the mold to obtain a product shape followed by cooling/molding. Using this method, molding in a short time of approximately 1 minute is possible by using a fiber impregnated with a resin in advance. This is a method of using a molding material called SMC, a stampable sheet and the like, and with such thermoplastic stamping molding, the fiber and the resin are made to flow inside the mold, which is associated with problems such that a thin-walled article cannot be prepared, fiber alignment is disturbed and cannot be controlled easily and the like.
Moreover, in Patent Literature 2, by forming a reinforcing fiber bundle into a parallelogram, a load borne by a chopped fiber bundle can be released from an edge portion of the reinforcing fiber to the periphery little by little toward the farthest portion in a fiber alignment direction, and a fiber-reinforced thermoplastic resin shaped product having excellent mechanical strength in which stress concentration is less likely to occur is proposed, while Patent Literature 3 proposes a stampable sheet which has improved wettability and adhesion on an interface between a reinforcing fiber bundle and a matrix resin, and excellent mechanical strength and molding fluidity by setting an aspect ratio of a section perpendicular to a bundle fiber axis of the reinforcing fiber bundle at 4 or more in a horizontal direction to 1 in a vertical direction.